Saturday, March 14, 2015

More work (and rework) on the SAC interface boards, plus continued work on the card reader.


The lever was not loosening any more with repeated manual exercising, which suggests that I may be forced into a total teardown to make this right. The good news, sort of, is that after some study of the manuals last night, I realized that this error itself should NOT cause either of the reading errors I saw. The effect is for one of the two rollers for drive wheels to remain disengaged. The only time it needs to be engaged is when we are punching cards.

Since there was a lot of crud in the throat of the punch unit and along the bed, I will make a working assumption that my problems came from the drag, which should be gone now. I temporarily reinstalled the punch unit, in order to work more on the reading side of the 1442.

What I found was that the drag from the bad roller pivot was enough to slow the good one down. I removed the pushrod to the bad pivot, which remains up and out of contact, allowing the good pivot to work as intended. However, this was not a usable change because the bad pivot does ooze downwards, even if it is too slow for normal operation, it is fast enough to jam the path within a few seconds. Back went the pushrod.

Reads now completed just fine, but the card wouldn't move through the punch station on the next cycle. There is some grease and gunk jamming the cards, but I worked that out using a few sacrificial cards.

Finally, the card reader is reading cards, getting occasional read checks due to draggy operation of the punch pivot, but mostly reading just fine and transporting cards right to the cornering station. If I release a bit of drag by slightly lifting the plastic cover on the cornering station, cards leap right to the stackers just as they should.

The punch pivot is exercised every time I operate the card feed cycle, so running the mechanism with the NPRO key will be enough to work it dozens to a hundred times for a reasonable length of NPRO operation. I can repeat this over and over until the pivot behaves as it should.

I will try adjusting the cover height for the cornering station to see if I can get the reader to be more dependable moving cards into the stackers, although I won't work on this today because the daylight is running out.


I spent the first hour this morning going over all the documentation and diagrams of the connections to be sure I had everything straight for what signal from the 1130 goes to a specific card, from the card to a cable wire, from that wire to a connector on the fpga, and from that connector to an FPGA logical pin. This will greatly improve the quality of my work going forward.

I sat down and finished wiring another six signals to the FPGA cable, making board two complete. As I got ready to move to board one to hook up another twelve signals, I had a sudden thought. Remembering that my first check of the driver circuits didn't look successful, while looking at the circuit board, I realized that I had taken as gospel the printed solder mask that had the cathode band of the diode marked.

Since these are polarized components, they have to fit in the right orientation, but I just couldn't imagine that the preconfigured artwork in the printed circuit board tool I used, DesignSpark, would print this incorrectly. Further, each time I inserted and soldered the diodes, I mentally saw the cathode marking as the anode, since the schematic symbol for a diode is a triangle for the cathode and a bar across the tip of the triangle for the anode. The bar looked like the marking, but of course the bar on the diode is the cathode, not the anode. I should have caught the error on the silkscreen on my boards right away.  observe my reaction

I now had to rework all four boards, removing 41 diodes which are backwards and installing 41 new diodes in the correct orientation. Made much more fun because the boards are in-place, mostly wired and therefore access is limited.

On some of the diode repairs, the signal trace from the PCB is no longer contacting the component lead. In most cases I could reflow some solder at the junction and restore connectivity, but in other cases I had to tack the FPGA cable wires to the diode when necessary.I am checking continuity of each end of the diode after it is replaced and handling problems appropriately.

I found that board two has no continuity from the ground pin on the power header to the ground plane - very odd indeed, must be a solder flow failure inside the via going down to the ground layer. I tried reflowing solder on the pin but it didn't improve. Fortunately, I have several larger than normal pads at vias bringing ground up from the ground plane to some component, which means I can tack a wire onto one of the via pads to get ground onto the board.

As of the end of day, I have all four boards wired to the FPGA board ribbon cables, but haven't installed the 50 pin connectors on the ends of those cables yet. I have the fix to the ground on board two, one new connection to a pin on board four, and the signal ground bus to connect to the box ground. More importantly, my original two ideas for how to mount the circuit cards aren't working out. I have a bit of thinking to do.


Today I checked out the circuit to the outdoor outlet on the side of the house delivering 240/120 20A, which I will plug into when I am powering the shed. My goal is to get it verified so I can move some gear, such as my 029 keypunch, into the data center shed in order to make more working room in the garage.

I am not happy with the current breaker, as it is not a GFCI type which is required by electrical code and of course required to be safe with outdoor lines. The correct breaker should arrive tomorrow afternoon, my target for powering up the shed. 

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